The present invention relates to an apparatus and a method for improved gas-solid (or fluid-solid) contacting. More specifically, the present invention relates to a spouted bed containing a novel draft tube geometry to improve the ease of scaling up of the bed. Our improved spouted bed and method allow for applications over a range of particle size, temperature, and vessel geometry.
Spouted beds have the potential to be a near-ideal solid-gas contactor, especially with a few proposed design changes to accommodate a wide range of applications. An example use of a spouted bed is the desorption of mercury species from pulverized coal into a hot gas stream. However, the inlet diameter of prior-art conical-cylindrical spouted beds must be less than about 25 times the particle diameter for good spouting flow behavior. For pulverized coal (diameter roughly 100 μm), this would lead to an unacceptable scale-up restriction for a coal-fired power plant. The addition of a draft tube in the spouted bed has been shown to alleviate this issue and assist in increasing throughput.
A Venturi draft tube spouted bed (VDTSB) can be used to enable equipment scale-up with small particles and to improve heat and mass transfer by increasing the material turnover rate. In a spouted bed, the solids are quickly carried up in a high-velocity gas jet, ejected in a fountain in the freeboard region of the bed, and slowly return back to the bottom before repeating the cycle. Mass transfer is poor in the annular portion of the bed that is settling back to the bottom, and therefore it is desirable to shorten that portion of the cycle and increase the turnover rate and exposure to the jet and fountain where mass transfer is high.
The VDTSB incorporates an eductor into the draft tube to provide suction at the bottom of the bed to better entrain material and reduce the propensity for bridging and dead zones at the bottom of the narrow conical section. This is done with a Venturi shaped nozzle in the inlet of the draft tube. Low pressure, high velocity fluid provides suction on the surrounding particles as it enters the Venturi throat which aids in the entrainment of the particles of the fluid stream. The Venturi nozzle section contains a converging section, a throat, and a diverging section, where the low pressure, high velocity fluid/particle mixture recovers its original static pressure and velocity. The main advantages of this approach are robustness, process scalability, and compatibility with a wide range of solid particle diameters, and enhanced heat and mass transfer.
Spouted beds are a way of contacting relatively coarse particles with a jet of fluid (usually air) that are characterized by relatively low pressure drop, good solid-gas contacting leading to good heat and mass transfer, and low tendency for segregation of particles based on size (and shape). Spouted bed vessels are usually cylindrical with a truncated cone at the bottom. An upward jet of fluid enters the tip of the cone. The high velocity in the jet entrains the particles in a central upwelling spout region. The particles in the spout are carried up beyond the bed in a fountain and then rain back down on the dense annular bed region surrounding the spout. The particles (and some gas) are transported downward, countercurrent to the spout, in the dense annular region before being re-entrained in the spout to repeat the cycle. The spouted bed can be operated in a batch mode or continuously with an inlet and outlet arranged (usually) with the inlet in the freeboard region above the bed and the outlet either in the lower conical region or at the level of the settled bed.
The presence of a draft tube encourages spouting and eases scale-up to larger vessels. The Venturi draft tube (VDT) further encourages spouting, eases scale-up, reduces the particle cycle time, and reduces dead zones in the conical region known to exist at larger scales thereby narrowing the cycle time distribution (CTD).
It is well known through many empirical and analytical studies of spouted bed hydrodynamics that the ratio of the inlet diameter Di to the particle diameter dp cannot exceed about 25 for stable spouting, Dj/dp<25. Furthermore the optimum ratio of vessel diameter to inlet diameter Do/Di is said to be 6 to 10 with a maximum of 12 for stability and to avoid dead zones (Epstein, N.; Grace, J. R., Spouted and Spout-Fluid Beds: Fundamentals and Applications. Cambridge University Press: 2010). Combining those two constraints yields:
            D      c              d      p        =                              D          c                          D          i                    ×                        D          i                          d          p                      ≤    300  Therefore the diameter of the bed should not exceed 300 times the particle diameter. This observation has been borne out in previous studies of industrial scale spouted beds; spouted beds are typically used for particles in the 1-3 mm range and almost never exceed 1 m in vessel diameter. Because of this limitation, industrially relevant spouted beds are limited to fairly coarse particles.
Draft tubes have been shown to ease the scalability constraints for spouted beds. Chandnani and Epstein (Chandnani, P.; Epstein, N. In Spoutability and Spout Destabilization of Fine Particles with a Gas, Proceedings of Fluidization V, 18-23, May 1986; Engineering Foundation: 1986; pp. 233-240) showed that stable spouting could be achieved with a draft tube spouted bed (DTSB) with a Dj/dp ratio up to 66.7, a nearly three-fold improvement over the conventional spouted bed without a draft tube. Nevertheless, further improvements are needed to process fine coal particles and the like. Altzibar et al. (Study of the Minimum Spouting Velocity in a Drain-tube Conical Spouted Bed, The 13th International Conference on Fluidization—New Paradigm in Fluidization Engineering, 2010) noted that:                A crucial parameter that limits the scaling up of spouted beds is the ratio between the inlet diameter and particle diameter. In fact, the inlet diameter should be smaller than 20-30 times the average particle diameter in order to achieve spouting status. The use of a draft-tube is the usual solution to this problem. Nevertheless, solid circulation, particle cycle time, gas distribution and so on, are governed by the space between the bottom of the bed and the draft-tube.        
The VDTSB has improved hydrodynamics in the entrainment zone in the base of the cone between the bottom of the bed and the draft tube. The concept essentially introduces an eductor into the draft tube. The jet of the spouted bed is used as the motive fluid in the eductor and the Venturi geometry in the draft tube accelerates the fluid (with minimal total pressure loss) to create additional suction in the entrainment zone. The additional suction draws particles radially inward to reduce the cycle time and eliminate dead zones. Both effects ease scale-up, which is limited by the appearance of dead zone that broaden the CTD and long cycle times that before now have led to unacceptable hold up of material in the dense annular region where mass transfer is slow.
A currently preferred embodiment of our invention contemplates that the VDTSB can be used for thermal desorption of mercury from coal. Bench-scale analytical mercury speciation studies have demonstrated thermal desorption; however, this method has not been applied to industrial-scale systems to pre-treat coal. A specialized desorber design would be required with high throughput, tolerance for a fairly wide particle size distribution, good heat and mass transfer, and robustness. A Venturi spouted bed desorber tailored for coal feedstocks could achieve conditions that promote thermal desorption of mercury species (primarily occurring in the range 170-300° C.) and meet the aforementioned requirements. Pulverized coal, which ranges in size from 10 to 500 μm and is usually less than 100 μm (Jiménez, S.; Ballester, J., Study of the Evolution of Particle Size Distributions and its Effects on the Oxidation of Pulverized Coal. Combustion and Flame 2007, 151 (3), 482-494). For the prior art spouted beds, this particle size would limit the diameter of the spouted bed to be less than 30 mm. Clearly, this limitation on the scale of prior art spouted beds makes them impractical for coal-fired boilers typically consuming coal at a rate on the order of 10 metric tons per hour or more.
An object of the present invention is to increase the throughput and scalability of spouted bed systems by improving radial solids transport within the entrainment zone of the draft tube by incorporating a Venturi nozzle.
A further object of this invention is to improve the solids-turnover rate with tailored spouted bed and draft tube geometries for feedstocks with a wide particle size distribution.
The present invention improves on conventional spouted beds by incorporating a draft tube with a converging-diverging Venturi nozzle to provide suction near the draft tube inlet and increase the flow rate of solids into the high-velocity motive fluid stream with a consequent reduction in the propensity for dead zones at the bottom of the narrow conical section.
The present invention adapts to different particle sizes, flow rates, and feedstock with the ability to quickly interchange nozzle and draft tube diameters. The length between the nozzle inlet and the bottom of the draft tube is “tunable”, i.e., selectively variable to accept a wide range of particle sizes, particle shapes, and particle material properties.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description when considered in conjunction with the accompanying drawings and non-limiting example herein.